In order to evaluate the energy transmit efficiency of main bearing gearbox of shield/TBM quantitatively, a reference value E (efficiency of energy output) is calculated and defined theoretically based on 3 variables, i.e., torque, rotational speed and driving current of the cutting wheel. Its feasibility has been verified in the research practice about prolonging the service life of shield/TBM. The parameter monitoring is carried out on working condition of shield/TBM components such as main bearing gearbox. The E parameter is worth developing and popularizing.